In a study comparing the Bactec 9240 (Plus Aerobic/F and Anaerobic/F bottles, containing resins; Becton Dickinson, USA) and the Vital (standard aerobic and anaerobic bottles, with no additives; bioMérieux, France) blood culture systems, 6456 sets of four bottles of 9660 blood cultures submitted were evaluated. There were 531 clinically significant isolates from 795 positive blood cultures. Of the 531 positive blood cultures, 355 were positive in both systems, 141 with the Bactec 9240 alone, and 30 with the Vital alone (p < 0.001); five were not detected by either system. The average time to detection of positive cultures for the matched sets was 10.65 h and 18.41 h by the Bactec 9240 system and the Vital system, respectively. The false-positive rate per bottle was 0.65% in the Bactec 9240 and 0.71% in the Vital. The rate of false-negative pairs (i.e., major errors) was very low (0.12% for the Bactec 9240, 0.19% for the Vital) and not significantly different between the two systems. The striking differences in recovery of microorganisms may be due to the presence of resins in the Bactec medium. However, the observed superiority of the Bactec 9240, even for patients not receiving antibiotics, suggests that resins adsorb other inhibitors present in patients' blood.